The disclosed device relates generally to fastening devices, and more particularly to fastening systems for use in the assembly of materials in applications having strict installation requirements, such as aircraft wings. For purposes of this disclosure, fasteners comprise a shank having threaded portions and a grip portion which has no threads. One such fastener has threads on opposite ends, with a grip portion between the threaded ends of the shank. This type of fastener may be utilized to attach two work pieces together, where nuts are made up on each of the threaded ends, as illustrated in FIG. 1. FIG. 1 depicts an idealized installation in which none of the threads of the fastener enter the bores of either work piece. FIG. 2 depicts a realistic installation in which some of the fastener threads enter into the bore of a work piece. The installation depicted in FIG. 2 illustrates the problems which are commonly presented by this type of fastening system. The intrusion of a portion of the threaded section into the bore of the work piece may damage the bore of the work piece by reaming out the bore when the work piece is vibrating.
Another installation problem may be detected by comparing the idealized installation depicted in FIG. 1 with that shown in FIG. 2. FIG. 1 generally shows about the same length of thread above each of the nuts. An idealized installation will have approximately two full threads of the shank extending beyond the top of each nut. If less than two full threads extend beyond the top of a nut, it may not be possible to apply the proper pre-load to the shank, or the nut may not be securely fastened. If excess threads extend beyond the top of the nut, it may also indicate that the threads of nut are entering the transition zone of the fastener shank between the threads and the grip length, where the fastener threads may be incomplete. In addition excess fastener extending past the top of the nut may interfere with the movement of nearby moving structures.
Thus fastening systems utilized for critical service are typically subject to three primary requirements: (1) the fastener threads are not to extend within bore, or grip area, of the work piece—the grip area should only be contacted by the unthreaded portion of the shank section of the fastener; (2) no shanking, whereby the nut cannot run so far down the threads of the fastener that some of the nut threads enters the thread transition zone of the fastener; and (3) there must be sufficient thread protrusion above the top of the nut, whereby a predetermined amount of thread length must protrude completely through the nut to ensure complete nut engagement.
Unfortunately, with the type of fastener system depicted in FIGS. 1-2, it is very difficult to install the system with the required preload, while maintaining the grip length of the fastener within the work piece bores and obtaining the proper thread extension beyond the tops of the nuts. As the fasteners are preloaded to the necessary torque, typically with the shank being retained in a fixed position by a tool either inserted into a broached end or attached to tooling at the end of the shank, it is not uncommon to pull some of the threads of the fastener into the bore. This is particularly true because most applications do no allow the grip length of the fastener to be observed by the installer as the fastener is installed, so the installer is not typically aware that threads are being pulled into the bore of the work piece. Thus an installer is often faced with two competing objectives—obtaining the proper preload while simultaneously trying to prevent fastener threads from entering the work piece bore. However, because the installation tools are usually automated to provide a specified preload and the grip length is not visible to the installer, it is common to simply apply the preload, such that an installation such as that depicted in FIG. 2 is an all too common result.